Forecast model · NOAA GFS forecast model, updated every 6 hours
Highs, lows and isobars
The lines are isobars, joining places with the same air pressure at sea level. They show the highs and lows that steer the weather: the wind circles around them and blows strongest where the lines crowd together.
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Air pressure
Air pressure is the weight of the air above you. At sea level it averages about 1,013 hectopascals (hPa). Weather systems change it by a few per cent: a strong high may reach 1,040 hPa, and a deep storm can fall below 950 hPa.
Lows
In a low, air near the ground flows in and rises. Rising air cools and its water vapour condenses, so lows bring cloud, rain and wind. Typhoons are among the deepest lows of all.
Highs
In a high, air sinks and spreads out near the ground. Sinking air warms and dries, so highs usually bring settled, clear weather: hot in summer, frosty on clear winter nights.
Reading the lines
The isobars are drawn every 4 hPa. The wind blows roughly along them, turned by the Earth's spin: anticlockwise round lows and clockwise round highs in the northern hemisphere, the other way round in the southern. The closer the lines, the stronger the wind.
Why 'sea level'
Pressure falls with height, so readings are converted to what they would be at sea level before they are compared. Over very high ground, such as Antarctica and the Tibetan Plateau, this conversion is unreliable, so no H or L is marked there.
Did you know?
The lowest sea-level pressure ever measured was 870 hPa, in Typhoon Tip over the western Pacific in October 1979.